Modulation of a signal entails the use of a carrier wave and manipulation of the carrier by changing either the amplitude, frequency, phase, or other wave characteristics to convey intelligence to a remote receiver which demonstrates the wave to determine the content of the source of modulation. A simple example of modulation is Morse Code on a telegraph which interrupts a carrier to form short and long pulses having meaning to the receiver. Similarly, telephones change the amplitude of a wave in proportion to movement of a coil in an electrical circuit. The spoken voice causes a diaphragm to be displaced by compression and rarefaction of the air near the mouthpiece. The affected wave travels through a telephone line to a receiver and causes a speaker to react to the wave driving the speaker and reproducing speech.
Digital modulation converts an input into a string of 1""s and 0""s and changes a wave in such a manner as to represent these two states which may be interpreted at a receiver as a binary sequence. Early digital modulation schemes carried one bit (either a 1 or a 0) with each cycle. This could be done using a variety of modulation techniques.
Multi-level amplitude modulation-based schemes, such as M-ary Quadrature Amplitude Modulation (QAM) and M-ary Vestigial Sideband (VSB), offer large potential gains in spectrum efficiency. When each transmitted symbol has only two possible transmission states, each symbol can only carry one bit of information. On the other hand, if multiple transmission states are available, each transmitted symbol carries log2 (M) binary bits, where M is the number of possible states. In other words, the number of bits n carried by a single symbol can be calculated from the number of possible states M with the following relationship 2n=M.
QAM is currently the most popular multi-level modulation scheme. 16 QAM (sixteen possible states for each symbol), 64 QAM (sixty four possible states for each symbol) and 256 QAM (256 possible states for each symbol) modulation systems are the currently used in point-to-point, high-capacity microwave systems. 512 QAM and 1024 QAM systems have been tested successfully and are beginning to appear. The currently used systems offer spectrum efficiencies of four (24=16) to eight (28=256) bits per cycle. With the current constraints on available spectrum, especially in mobile bands below 3 GHz, attaining spectrum efficiency is highly desirable.
M-ary QAM and other multilevel modulation schemes are used throughout the microwave bands for point-to-point fixed plant transmission. However, these modulation schemes have generally not been used in mobile radio communications. This is due largely to the need for higher signal-to-noise ratios and the commensurate higher power to achieve bit error performance. While much of this theoretical increase in required signal (and transmission power) can be negated by application of robust, digital error-correction schemes, there is a fundamental practical reason why QAM has not found support in the mobile radio communications application. Specifically, current multilevel modulation transmission techniques demand a lot of power. Mobile radio devices, however, have limited power. Consumers want small wireless phones that have a long battery life. To meet these important goals, the size of the mobile phone batteries is reduced and the power consumption of the phone is minimized. The minimized power is not sufficient to support current multilevel modulation transmission without adversely affecting battery life.
Current multilevel modulation demands a lot of power because linear amplifiers are used at the final RF stage. Multilevel modulation schemes, such as QAM, make slight changes in the transmitted symbols. The symbols are not simply xe2x80x9conxe2x80x9d or xe2x80x9coff.xe2x80x9d Very accurate, distortion-free transmission is required in order for multilevel modulation symbols to be accurately recognized at the receiver. Linear amplifiers at the final RF stage produce an amplified output signal which is directly proportional to the input signal. However, a linear amplifier operates in the linear area of the transistor power curve, between where the transistor is completely off and where the transistor is completely on. The transistor is always partially on. There is current flowing and heat dissipating even when no signal is being supplied. A substantial amount of energy is lost as heat, rather than producing radio frequency signals for the antenna.
Amplifiers are divided into classes. Classes A, B, and AB amplifiers are considered to be linear amplifiers. In view of their linear operation, the amplifiers of classes A, B, and AB have maximum power efficiencies of between 30% and 40%. Non-linear amplifiers, on the other hand, are not always partially on. They are either on or off. The amplifiers of Classes C, D and E are non-linear amplifiers. Non-linear amplifiers can achieve power efficiencies of between 60% and 90%. However, non-linear amplifiers produce an amplified output signal which may not be a proportional reproduction of the input signal. Thus, conventional wisdom has determined that non-linear amplifiers are not suitable for the required very accurate, distortion-free transmission of multilevel modulation schemes.
Since only linear amplifiers are used currently for M-ary QAM, the maximum efficiency currently attainable for M-ary QAM RF transmitters is between 30% and 40%. In mobile communications and satellite systems this demands two to three times the electrical energy battery capacity. Since battery capacity is somewhat related to battery size, this means larger batteries. Alternatively, for a given battery, conventional multilevel modulation schemes reduce battery life to one-half to one-third of the battery life for a modulation scheme having only two possible states. The impact on battery size and battery life has been one of the major limitations in the use of multilevel modulation schemes for mobile and satellite communications.
It is one possible object of the invention to enable to use of non-linear amplifiers for multilevel modulation schemes.
One way to possibly accomplish this goal is to select and activate at least one amplifier from a first bank of non-linear amplifiers. At least one amplifier is selected from a second bank of non-linear amplifiers. The number of amplifiers selected from the first and second banks remains constant. Using the selected amplifiers, first and second components of a phase modulated digital signal are amplified using the first and second banks, respectively. The amplifiers banks introduce amplitude modulation to each signal component by activating the selected amplifiers for the duration of a digital symbol. The outputs produced by first and second banks are combined to produce an analog representation of the digital symbol.
The first component may be phase shifted by 90xc2x0 with respect to the second component. According to one alternative at all times during operation, one and only one amplifier is selected from each of the first and second banks of amplifiers. The first component may be an in-phase component of the digital symbol and the second component may be a quadrature phase component of the digital symbol.
There may be four amplifiers in the first bank of amplifiers and four amplifiers in the second bank of amplifiers. If there are m amplifiers in the first bank of amplifiers and n amplifiers in the second bank of amplifiers, then each QAM symbol produced by the combined array may carry r bits of data such that 2r=(2m*2n).
Each amplifier of the first bank may have a different gain characteristic, and each amplifier of the second bank may have a different gain characteristic. In this case, the individual gain characteristics of the first bank may be the same as the individual gain characteristics of the second bank. The gain characteristics may be selected so as to provide an equal gain intervals between the amplifiers of each bank. In this manner, the individual amplifiers from the first and second banks may be chosen for operation based on their gain characteristics.
An amplifier device has first and second banks of non-linear amplifiers, a selection unit and a combination unit. The selection unit selects at least one amplifier from each of the first and second banks of non-linear amplifiers such that for each bank, the selected amplifier at least one amplifier produces an amplified output from a different component of a digital symbol, and such that the number of amplifiers selected remains constant. The combination unit combines the amplified outputs from the first and second banks. The first and second banks may be formed on an application specific integrated circuit chip.
A wireless telephone may be formed from the amplifier device, a telephone body, a speaker, a microphone, an antenna, and a transceiver. The amplifier device serves as the radio frequency amplifier within the transceiver.
According to another aspect, a modulation device has a Q-phase premodulator, an I-phase premodulator to respectively produce Q- and I-phase components. At least one non-linear Q-phase amplifier amplifies the Q-phase component and produces an output. At least one non-linear I-phase amplifier amplifies the I-phase component and produces an output. A combiner combines the outputs of the Q-phase and I-phase amplifiers.
The Q-phase component and the I-phase component may contain substantially no amplitude information. In this case, amplitude information is added in the Q-phase and I-phase amplifiers.
The combiner may produce an output signal that is fed to an antenna device with no intervening linear amplifiers. The combiner may produce an output signal that is sufficiently amplified for wireless transmission. The antenna device may be directly connected to the combiner. The combiner can comprise a simple impedance matching circuit with a direct connection to the antenna.